Increase quality of water render
I'm not entirely sure what part of your render you are asking about, but here are some possibilities:




This appears to be caused by internal reflection of a light source (judging from the lighting, I would say the light source is behind the camera and a little to the left). Try adjusting the lighting in your scene and see if it improves.
The fluid simulation appears to be rendered at a low resolution. Try baking at a higher resolution (e.g. start with 100). If you did bake at high resolution, ensure that the Render display option is set to Final.
It's hard to say what's causing this without looking at your file, but a couple of possibilities are:


The geometry of the orange mesh. Ensure that the normals in that area are facing the right way, check for overlapping faces, ensure that there are no holes in the mesh, etc.
Bounces. Try increasing the number of Bounces in Render settings > Light paths (A fast way to increase all bounce related settings is with the presets, try the Full Global Illumination preset). This will increase the overall accuracy of the render as well, so you probably want to increase them anyway. Note that more bounces will slow down your render.

To help decrease fireflies, see this question. Some techniques mentioned there are:


The Clamp option (may or may not be very helpful in this scene, however I thought I would mention it). From the wiki:


  This option will clamp all samples to a maximum intensity they can
  contribute to the pixel, again to reduce noise at the cost of
  accuracy. With value 0.0 this option is disabled; lower values clamp
  more light away. 
  
  If the image has fireflies, there will be samples that contribute very
  high values to pixels, and this option provides a way to limit that.
  However note that as you clamp out such values, bright colors in other
  places where there is no noise will be lost as well. So this is a
  balance between reducing the noise and keeping the image from losing
  its intended bright colors.

Render stacking. See the following posts:


Stacking cycles renders in the compositor?
Creating renders from Image Stacking using different sampling levels
How can I pause a render in progress and then resume it at a later time?

Fake refractive caustics
Filter Glossy
No Caustics (definitely not useful in this scene, as you need the caustics for light to reach the fruit, but again I thought I would mention it anyway)



See the manual page on reducing noise.